Method of fabricating an assembly comprising an anode-supported electrolyte, and ceramic cell comprising such an assembly

ABSTRACT

Electrochemical cell on the basis of electrode-supported thin electrolyte includes an anodic support ( 1 ) consisting of a nickel/zirconia material and an electrolyte layer ( 3 ) disposed that includes zirconia thereon and a cathode layer ( 6 ) which is in turn disposed thereon. To improve the quality of the cell and to increase the output it is proposed to arrange an auxiliary or intermediate layer ( 2 ) between the electrolyte layer and the anodic support. This auxiliary layer consists of nickel and zirconia particles which are substantially smaller than the particles of the support. Then an electrolyte layer ( 10 ) and, on the other side of the anodic support, a current collector layer are applied. None of these layers are sintered. Only after an assembly obtained in this manner has been obtained is a sintering treatment carried out.

The present invention relates to a method of fabricating an electrodesupport, comprising the steps of providing an assembly comprising ananodic support made of nickel/zirconia material, of applying to one sideof that anodic support an anode auxiliary layer comprising a mixture ofnickel oxide and YSZ particles on top of which an electrolyte layercomprising YSZ particles is applied.

Such a support is disclosed by DE-19819453 A1. There, an auxiliary layeris applied to a sintered anode support, and after a prior heat treatmentan electrolyte layer is put onto the auxiliary layer. The assemblyconsisting of anode support, auxiliary layer and electrolyte layer isthen sintered at 1400° C. A cathode is then applied and a furthersintering treatment is carried out.

It is the object of the present invention to provide a method whichincorporates the step of applying a current collector layer disposed onthe other side of the anode support.

With a prior art method, sintering of the anode support and applicationof an auxiliary layer and/or electrolyte layer after this has beensintered is followed by the application of a current collector layer,after which the assembly is subjected to yet another sinteringoperation.

It has been found that the output of an electrochemical cell comprisingsuch an assembly is not optimal, and it is the object of the presentinvention to improve the output of such a cell and its reliability. Thisobject is achieved for an above-described method in that the step ofproviding the assembly is followed by subjecting said assembly to asintering treatment, the anodic support and the various layers and/orcombinations thereof of the assembly being in a nonsintered state priorto said sintering treatment.

It is assumed that the inventively improved characteristics of anelectrochemical cell fabricated with the assembly according to theinvention are achieved because none of the layers of which the assemblyis composed has been subjected to a sintering treatment before thesintering treatment of the entire assembly is carried out. Inparticular, this allows the so-called sintering shrinkage to be avoided,i.e. the shrinkage which results in prior-art assemblies from repeatedsintering of specific layers and one-off sintering of other layersdisposed thereon. Moreover, the number of sintering steps is decreased,thereby reducing production costs. According to an advantageousembodiment of the invention, the current collector layer is also appliedbefore the assembly is sintered and is sintered together with theassembly.

According to the invention, the above-described sintering treatmentpreferably takes place at a temperature of between 1300 and 1500° C.

To impart some strength to the anodic, preferably nonsintered, support,it is possible, according to the invention, to subject this to a heattreatment at a temperature of between 900 and 1100° C., the detailsdepending on the materials used and technical possibilities. That is tosay that in principle it is possible to dispense with such a prior heattreatment of the anodic support.

Apart from the above-described layers, the assembly can moreovercomprise a cathode layer and/or additional electrolyte layer orelectrolyte auxiliary layer, respectively.

The above-described assembly allows the operating temperature of a solidoxide fuel cell to be further reduced. At present, a temperature rangeof 700-800° C. is considered desirable. This low temperature allows thestack and system components of the cell to be constructed more cheaply.That is to say, the steel grades used can be less expensive ferriticstainless grades. Moreover, components customary for installations inthe prior art can be employed, and the service life of the variouscomponents and consequently of the cell can be extended considerably.

However it has been found that at a low operating temperature theefficiency of the solid oxide fuel cell decreases. The reason for thisis that the voltage losses across the cell increase with fallingtemperature. In the prior art it has been proposed to reduce thethickness of the electrolyte to values below 40 μm, as a result of whichthe voltage losses across the ceramic cell decrease and lower operatingtemperatures can be achieved. As thin electrolyte layers of this typehave negligible mechanical strength, electrode-supported solid oxidefuel cells have been proposed. In such an arrangement, an electrolytematerial in the form of a thin layer is, for example, applied to ananodic support which has been sintered. The above-described publicationdescribes such an anode-supported thin electrolyte layer. Starting fromnickel oxide and YSZ (yttrium-stabilized zirconia), a suspension isprepared which is shaped as desired by uniaxial pressing, followed bysintering. The electrolyte layer is applied directly to the anodesubstrate thus prepared.

The application of an auxiliary or intermediate layer consisting ofNiO/YSZ was found to result in filling of the larger pores of the anodesubstrate, thus producing a very flat surface of the anode support. Thiscomparatively dense layer is comparatively thin, i.e. its effect on thediffusion of gases is comparatively small. The underlying thicker layerhas greater porosity, which means that the movement of the gases is nothampered. That is to say that according to the invention a design isobtained which on the one hand has a very flat boundary without largecavities but on the other hand is sufficiently gas-permeable to permitmigration of gases. According to an advantageous embodiment, theparticle size chosen for the intermediate layer such that the surface ofthe anode support only has pores smaller than about 1 μm and a minimumof defects or even is free from defects. This allows the electrolyte tobe applied under optimal conditions to the support with the auxiliarylayer. The electrolyte is thus fabricated free from defects.

According to an advantageous embodiment of the invention, the meandiameter of the pores of the auxiliary layer is below 0.5 μm, while theunderlying thicker support has pores having a mean diameter of between0.5 and 3.0 μm.

The porosity of the auxiliary layer in particular is about 40 vol %,while the porosity of the support is between 40 and 60 vol %.

The anodic support is fabricated, for example, by a suspension beingprepared, starting e.g. from nickel oxide and zirconia material, andthis being given the desired (green) shape, for example by means ofsheet casting or extrusion, and possibly some strength being impartedthereto by heat treatment in the anodic support. This heat treatment canbe carried out in any manner known in the prior art, at a temperature ofbetween 900 and 1100° C. over a period of from 1 to 8 hours. Thistreatment does not comprise any sintering. Sintering is carried out at ahigher temperature.

Next, the anode auxiliary layer is applied. The technique of applicationto the anodic support can comprise any method known in the prior art.Since a comparatively thin layer has to be applied, the screen printingtechnique is particularly suitable. The layer thickness of the auxiliarylayer can be between 3 and 20 μm. Next, a layer of YSZ of the samethickness can be applied which, after the heating step, acts as theelectrolyte. Next, the assembly thus obtained can undergo a sinteringtreatment at a temperature of between 1300 and 1500° C. The sinteringtime can be about one hour. Then the cathode is embodied. Cosintering isan option. The invention moreover allows fabrication costs to be keptwithin limits and the production to be scaled up in a simple manner.

To improve the current collection on the anode side, a separate currentcollector is provided. This current collector can comprise nickelmaterial. Such a current collector can be embodied by a nickel oxidelayer having e.g. a layer thickness of 10-40 μm being applied to theanode before final sintering. During operation in an electrochemicalreactor, nickel oxide will be converted into nickel. Apart from actingas a current collector, the nickel layer prevents nickel depletion fromthe anode support during final sintering. This is because at thecomparatively high sintering temperature (1300-1500° C.) nickel oxidehas been found to evaporate and to migrate towards the sintering platesbetween which sintering takes place. Placed against said currentcollector is a further current collector made of metallic material, suchas a gauze or plate material. The sequence of the application of thevarious layers to the anode support can be modified as desired.

One example of the invention is illustrated in more detail withreference to the drawing. The single FIGURE shows an anode-supportedelectrolyte. The anode support is indicated by 1 and consists of anickel/zirconia layer. The thickness of this layer is between 200 and1000 μm. This layer serves to impart mechanical strength and should bereadily gas-permeable. Moreover, because of its thickness it should lenditself to being fabricated in a comparatively inexpensive manner, forexample by means of sheet casting or extrusion. Other metal-ceramiccompositions are conceivable, copper and cobalt being potentiallysuitable as metals and titanium oxide, aluminium oxide and magnesiumoxide being potentially suitable as ceramic materials. Optionally, asteam-methane reformer catalyst can be present, which converts steam andmethane into carbon monoxide and hydrogen. 3 indicates the electrolytelayer, consisting of 8 YSZ. In general, the electrolyte should consistof oxygen ion-conducting ceramic materials such as ceria, zirconia andperovskites, which can be doped e.g. with lanthanide or alkaline earthmetals. This layer 3 has a thickness of between 3 and 40 μm. The anodeauxiliary or intermediate layer is indicated by 2. This consists ofnickel and a ceramic material such as zirconia, ceria and the like. Hereagain, variations are possible, and in general a combination of a metaland oxygen ion-conducting ceramic will be used, consisting e.g. oflanthanide- or alkaline earth metal-doped ceria, zirconia orperovskites.

4 indicates a current collector layer. In the operational stage, thisconsists of nickel and has a thickness of between 4 and 40 μm.

The current collector preferably includes a contact layer made of somemetal. If nickel is used, the evaporation of nickel oxide from thesupport layer 1, the anode support, during sintering will be inhibited.

Layer 5 is an additional electrolyte layer or electrolyte auxiliarylayer which can be applied directly on top of electrolyte layer 3. Thislayer consists of lanthanide- or alkaline earth metal-doped ceria. Theuse of this additional electrolyte layer permits mixed-conductivityperovskite materials such as La_(0.6)Sr_(0.4)Fe_(0.8)Co_(0.2)O₃ to beemployed which normally react with a zirconia electrolyte but do notreact with a ceria electrolyte. Layer 5 can be sintered at 1300-1500° C.together with the anode support and the anode auxiliary layer andelectrolyte layer disposed thereon and the current collector layer.Alternatively, this layer 5 can be sintered in a separate step after theassembly consisting of the anode support, anode auxiliary layer,electrolyte layer and current collector layer has been sintered. Layer 6is the cathode layer, which can consist of a bilayer cathode, consistingof 1) a mixture layer of (La,Sr)MnO₃ (LSM) and zirconia on top of this,and 2) a current-collecting layer consisting of LSM if electrolyte layer3 only is used. In the case of electrolyte layer 5 being used as wellthere is the option of employing mixed-conductivity perovskite materialssuch as La_(0.6)Sr_(0.4)Fe_(0.8)Co_(0.2)O₃ as the cathode material.Layer 6 can likewise be sintered directly with the above-describedassembly and also, in a separate step, be sintered after theabove-described assembly has been sintered. Those skilled in the art,having read the above, will immediately be able to think of variationswhich are obvious and are within the scope of the appended claims.

What is claimed is:
 1. Method of fabricating an electrode support,comprising the steps of providing an assembly comprising an anodicsupport made of nickel/zirconia material, of applying to one side ofthat anodic support an anode auxiliary layer comprising a mixture ofnickel oxide and YSZ particles on top of which an electrolyte layercomprising YSZ particles is applied, wherein in that the step ofproviding the assembly is followed by subjecting said assembly to asintering treatment, the anodic support and the various layers and/orcombinations thereof of the assembly being in a nonsintered state priorto said sintering treatment, characterised in that, said auxiliary layeris applied in a thickness of 3-20 μm by screen printing; a currentcollector layer being provided, which is disposed on the other side ofthe anodic support, including metal particles and in a nonsintered stateprior to said sintering treatment, and wherein a nickel oxide layerhaving a thickness of 10-40 μm is applied to the inside of said anodicsupport.
 2. Method according to claim 1, wherein said sinteringtreatment is carried out at a temperature of between 1300 and 1500° C.3. Method according to claim 1, wherein the step of providing an anodicsupport comprises a heat treatment of said anodic support at 900-1100°C.
 4. Method according to claim 1, wherein said assembly comprises acathode layer.
 5. Method according to claim 1, wherein said assemblycomprises an electrolyte auxiliary layer disposed between electrolytelayer and cathode.
 6. Method according to claim 1, wherein the step ofproviding the anodic support comprises the fabrication, by means of thetape casting technique or extrusion, of a plate having a thickness ofbetween 0.3 and 1.0 mm.
 7. Method according to claim 1, wherein saidauxiliary layer is applied as a suspension to the support by means ofscreen printing.
 8. Method according to claim 1, comprising a furtherelectrolyte layer on said electrolyte layer which comprises YSZparticles and is cosintered therewith.
 9. Method according to claim 8,wherein said further electrolyte layer comprises ceria doped withlanthanide or alkaline earth metals.
 10. Method according to claim 1,wherein the pores of said auxiliary layer have a mean diameter of lessthan 0.5 μm.
 11. Method according to claim 1, wherein the pores of saidanodic support have a mean diameter of 0.5-0.3 μm.
 12. Method accordingto claim 1, wherein the porosity of said auxiliary layer is about 40 vol%.
 13. Method according to claim 1, wherein the porosity of said supportis 40-60 vol %.
 14. Method according to claim 1, wherein the currentcollector layer comprises nickel.
 15. Electrochemical cell comprising anassembly obtained by means of the method according to claim 1.